The present invention relates to a heat exchanger and certain improvements thereto. In other aspects, the invention relates to the assembly of a heat exchanger and its components.
Heat exchangers, especially those utilized for steam generation, suffer from many problems. The problems encountered include corrosion phenomena, such as cracking, pitting, wastage, and denting of the tubes; mechanical phenomena, such as water hammer, vibration, cavitation, and tube splitting and fatigue; and functional phenomena, such as poor heat exchange, high pressure drop, and low throughput.
Heat exchangers for steam generation are designed to evaporate water. However, if conditions exist that cause a local region to dry out (i.e., all the water to vaporize), aggressive chemicals can concentrate and deposit in that region, harming metallic structures. In steam generators, the heating fluid passes on the inside of the tube bundle, and the fluid to be heated and vaporized flows around the outside of the tube bundle. Dry-out conditions commonly occur at the interfaces between the tubes and the support elements and also the tubes and the tube sheet are especially prone to dry-out conditions. Dry-out and high chemical concentrations in these crevices can cause tube denting and cracking and lead to premature failure of the tube bundle. Tube denting occurs when the carbon steel tube support element exhibits fast linear corrosion at the location where the tube passes through it. The corrosion product occupies a greater volume than the original support element metal and squeezes down on the tube, deforming it. If corrosion continues, gross deformation and eventual cracking of the support element and tube can result. Similar problems are seen in crevices between the tube and the tube sheet.
Dryout can often occur at the interface between the tube and sludge accumulations in the shell side of the heat exchanger. Sludge accumulations up to 12 inches thick have been measured on the tube sheets of operating vertically oriented steam generators. When phosphate water chemistries are employed, wall thinning of the steam generator tubes sometimes occurs in the sludge pile.
Tube vibration and collision can damage the tubes. Normally, clearance exists between the tubes of the bundle and the tube supporting elements. Such clearances are normally required for manufacture of the tube bundle. Fluid flowing parallel to, perpendicular to, or at some intermediate angle to the tubes induces tube vibration. This vibration may cause a tube to hit or slide against support elements and/or adjacent tubes, resulting in local wear damage. Because the movement between the surfaces is oscillatory and usually of small amplitude, the rubbing process is termed "fretting". Fretting causes tube metal loss, support element metal loss, and fretted regions are sensitive to fatigue cracking.
Water hammer can also damage the tubes of the bundle of a steam generator. Water hammer is thought to be caused this way:
(1) feed flow is interrupted, and susceptible regions fill with steam;
(2) when feed flow resumes, a volume of steam is trapped and rapidly collapses because of heat transfer to the colder feed water;
(3) the rapid collapse of the steam volume creates a pressure differential that accelerates a slug of water through the susceptible region; and
(4) the accelerating slug of water impacts (hammers) a barrier.